— 70 — 



frequency of Conferva (see the chart on p. 66). Were it not 

 for this the sunny Mardi of 1907 and 1010 would probably not 

 defer the maximum of tins genus to May. But Mardi is a criti- 

 calperiod, and, if it proves unfavourable to the development of 

 the Conferca, Oedogonium and Microspora will ohtain a 

 foothold, and Conferva Ls excluded until they hâve decreased 

 or the Phanerogams hâve multiplied sufficiently to affbrd enough 

 snbstratum for ail titrée gênera. The keenest eompetitors are 

 probable Conferva and Microspora, since conditions which 

 favour Oedogonium are adverse to Cou ferra. The struggle 

 between Microspora and Conferva is weD seen in 1908, when 

 tlie appearance of the Conferva in quantitj in Mardi, is follow- 

 ed bv a réduction in the amount of Microspora in the followmg 

 month. 



In the three years(1906-190S) in which Ophiocytium (mainly 

 0. cochleare)was well represented in tlie pond, it was présent in 

 small quantity practically ail the year round and in eadi of the 

 three years beeame important in Mardi or Àpril, i e. at about 

 the time tliat the filamentous algal flora becomes well developed. 

 Whereas it soon decreased in amount in 1906 and 1907. itper- 

 sisted in some quantity till the end of .Tune in 1908, a behaviour 

 which is quite identical with that of Conferva in that year. 

 Tins uniformity between thèse two allied forms is of considér- 

 able ïnterest, and the case of Ophiocytium may find the same 

 explanation as that put forward for Conferva (1) . 



The effect of sunshine is again rather obvious in the case of 

 Ophiocytium. Thus, in 1906 with a small amount of sunshine 

 in Mardi it persists in quantity into April, but is reduced after 

 that month with its exceptional sunshine. In 1907 a decrease 

 is seen as early as April as a resuit of the unusually brighl 

 Mardi. In August of 1907 Ophiocytium was much commoner 

 than usual at tins time of the year ; even in .Tune and .Tuly it had 

 notbeen as rare as in 1900. This again accords well with the 

 sunshine-data. which are as follows : 



June to 1906 1907 1908 



August < 2 > 765 hrs. 506 hrs. 658 hrs. 



(1) The more uniform perioilicity o( Ophiocytium, &s compared \\ itli Con- 

 ferva, is prol'a'ily in part due to the ahsence of compétition in the case ol" tlie 

 tonner. 



(2) The resuit remains the same, if \ve include May or exclude August. 



